This invention relates to alloys and more particularly to fusible alloys.
Fusible alloys are often used in applications requiring temporary support or anchoring of a component. For example, fusible alloys can be used to support thin walled tubing during bending. After the bending operation, the tube can be heated in an oil or water bath and the melted alloy removed. Similarly, a device or component can be anchored in place by casting melted fusible alloy around it. After the component has been worked on, or when the device needs to be reorientated, the alloy can easily be melted and the anchored item removed. The fusible alloy can be recycled.
One area where fusible alloys have found particular use is in lens blocking. During the production of an optical lens, the glass or plastic lens blank must be locked in position to permit accurate grinding and polishing. This is achieved by attaching the lens blank to a lens block. The lens block, which supports and anchors the lens, can then be clamped into the grinding and polishing machinery.
Before fusible alloys were available, molten pitch was used to fix glass lens blanks to the blocks. However, the pitch was applied at high temperatures, sometimes causing the lens to crack. Further, removal of the pitch required a lengthy cleaning process.
In comparison, fusible alloys can be used at lower temperatures and can be removed easily. The first step in such a lens-blocking process with fusible alloys is to affix the lens blank to the lens block. Next, melted fusible alloy is introduced into the block. The alloy is allowed to solidify as it contacts the block and the lens blank, fixing the lens blank in position. The lens blank is then ground and polished. To remove the lens, the block is struck sharply; the lens pops out cleanly, obviating the need for lengthy cleaning.
Both the lens blocks and the fusible alloy are recycled. The used blocks are heated in a tank of hot water melting the fusible alloy. The blocks can then be removed ready for new lens blanks. The melted fusible alloy collects at the bottom of the hot water tank where it can be drained off for re-use.
To be suitable for lens blocking, a fusible alloy should have a low melting point. The low melting point makes it easier to remove the alloy from used lens blocks; it also means that the melted alloy can be applied to cold lenses without cracking or otherwise damaging them. Alloys with melting points up to about 160.degree. F. can be used for blocking glass lenses. Plastic lenses, however, are much more sensitive and require alloys with melting temperatures below about 130.degree. F.
Two low melting point alloys commonly used in lens blocking are ASTM Alloy 136 and ASTM Alloy 117 (see ASTM Specification B 774 incorporated herein by reference.)
ASTM Alloy 136 is a eutectic with a melting point of 136.degree. F. and comprises 48.5-49.5% by weight bismuth, 17.5-18.5% by weight lead, 11.5-12.5% by weight tin and 20.5-21.5% by weight indium. One such alloy is Indalloy 136, manufactured by the Indium Corporation of America. Indalloy 136 comprises 49.0% by weight bismuth, 18.0% by weight lead, 12.0% by weight tin and 21.0% by weight indium.
ASTM alloy 117 is a eutectic with a melting point of 117.degree. F. and comprises 44.2-45.2% by weight bismuth, 22.1-23.1% by weight lead, 7.8-8.8% by weight tin, 18.6-19.6% by weight indium and 4.8-5.8% by weight cadmium. One such alloy is Indalloy 117 manufactured by the Indium Corporation of America. Indalloy 117 comprises 44.7% by weight bismuth, 22.6% by weight lead, 8.3% by weight tin, 19.1% by weight indium and 5.3% by weight cadmium. ASTM Alloy 117 has a low enough melting temperature to allow it to be used to block plastic lenses.
ASTM Alloy 117, however, suffers from the disadvantage that it contains cadmium. Cadmium is considered toxic by the EPA and other government agencies. The present OSHA standard for cadmium fumes is 0.1 mg/m.sup.3. However, the National Institute for Occupational Safety and Health has recommended even more stringent restrictions--namely, a maximum cadmium level of 0.04 mg/m.sup.3 to protect against the chronic and acute effects of cadmium fumes.
Cadmium can cause problems when used as a lens blocking alloy. If the alloy is overheated, cadmium may fume off from the alloy creating dangerous concentrations of cadmium. Further, if the hot water used to melt the fusible alloy out of the lens blocks is slightly acidic, then cadmium may dissolve in it. The cadmium-containing water is poisonous and great care and expense must be taken in its disposal.
Thus, it would be desirable to provide a low melting point cadmium-free alloy.
It would further be desirable to provide a cadmium-free fusible alloy suitable for lens blocking.
It also would be desirable to provide a cadmium-free fusible alloy with a melting temperature below 130.degree. F. suitable for blocking plastic lenses.